The recovery of elemental sulfur from sulfur containing gas streams by the Claus process is a widely practiced procedure. In this procedure elemental sulfur is produced by the well known Claus reaction as follows: EQU 2H.sub.2 S+SO.sub.2 .revreaction.3S+2H.sub.2 O (1)
Under normal circumstances, the feed gas to the Claus process contains a substantial portion of hydrogen sulfide which is partially oxidized to produce sulfur dioxide in an amount approximately satisfying the stoichiometric relationship indicated by reaction (1). Thus, sufficient oxygen is supplied to the hydrogen sulfide containing stream under combustion conditions to oxidize about 1/3 of the hydrogen sulfide to sulfur dioxide. The two reactants then produce elemental sulfur in a Claus reactor over a catalyst such as bauxite or alumina. This so called partial combustion process requires a relatively high hydrogen sulfide content normally on the order of about 50 volume percent or more. Where the hydrogen sulfide concentration is substantially lower than this value, it is difficult to maintain stable partial combustion under industrially feasible conditions. In the case of feedstreams containing hydrogen sulfide concentrations of less than 50 volume percent, it is a normal practice to employ the so called "split stream" Claus process. In this procedure approximately 1/3 or more of the feedstream is diverted to a combustion zone where oxidation of the hydrogen sulfide to sulfur dioxide is carried out in the presence of an amount of oxygen adequate to provide sulfur dioxide in sufficient amount as indicated by reaction (1). The remaining portion of the stream is not treated so that when the split portions are recombined and passed to the Claus reactor, the combined input stream contains the hydrogen sulfide and sulfur dioxide in the approximately stoichiometric amounts of two parts hydrogen sulfide for one part sulfur dioxide. For a further description of the Claus process reference is made to Kohl et al Gas Purification, Second Edition, Gulf Publishing House, 1974, pages 370-380 under the heading "The Claus Process", the disclosure which is incorporated herein by reference. The tail gases from Claus sulfur recovery units still include sulfur compounds which may be present in concentrations ranging from a fraction of a percent up to several percent. The principal sulfur compounds are hydrogen sulfide and sulfur dioxide. These may be present in the approximate proportions of 2/3 hydrogen sulfide and 1/3 sulfur dioxide as indicated by the stoichiometry of reaction (1) or, in some cases, substantial concentrations of other reduced sulfur compounds may be encountered. The reduced sulfur components typically encountered, in addition to hydrogen sulfide, include carbonyl sulfide, carbon disulfide, and organic thio compounds such as the mercaptans. While the principal oxidized or unreduced sulfur compound is sulfur dioxide, elemental sulfur in a fine mist and as vapor is usually present in the off gas to some extent. At this point in the processing the major components of the off gas are inert gas components such as steam, nitrogen and carbon dioxide.
In the current regulatory environment, the sulfur contaminants in the Claus unit off gas (tail gases) are well in excess of those which can be released to the atmosphere. While many procedures are available for reducing the sulfur content of the exhaust gas stream, one common procedure involves hydrogenating the unreduced sulfur values, principally sulfur dioxide, in the presence of a hydrogenation catalyst to convert substantially all of the sulfur values to hydrogen sulfide. Typical procedures employed in industry are summarized in Kirk Othmer, Encyclopedia of Chemical Technology, Third Edition, Wiley - Interscience, volume 22, pp. 276-282, the disclosure of which is incorporated herein by reference. As disclosed in Kirk-Othmer, the hydrogen sulfide may be absorbed by a solvent which has a high selectivity for hydrogen sulfide in the presence of carbon dioxide and the hydrogen sulfide then regenerated from the rich solvent and returned to the front of the Claus process.
As noted above, one relatively simple and straightforward technique for accommodating the treatment of relatively low sulfur content gas stream is the so called split-flow Claus process in which a portion of the feed is diverted from combustion and then recombined with the remainder of the feed. Other more sophisticated procedures have also been proposed. These typically involve a recycle step and may or may not involve split streams. One procedure which is said to be particularly applicable to hydrogen sulfide containing gases having up to 85% volume percent or more carbon dioxide is disclosed in U.S. Pat. No. 4,263,270 to Groenendaal et al. In this process, the feedstream is passed through a nonselective absorption and regeneration unit and then split, with one branch of the stream applied directly to a Claus reactor and the other bypassing the Claus reactor and applied directly to an absorption and regeneration unit which is selective for hydrogen sulfide in the presence of carbon dioxide. The tail gases from the Claus unit are applied to a hydrogenation reactor in which they are subjected to a reduction step for conversion to hydrogen sulfide. The reduced off gases from the hydrogenation reactor are then applied to the selective absorption regeneration unit and hydrogen sulfide gas recovered from the regeneration unit is applied to the split portion of the stream which is fed directly to the Claus plant.
Another process involving the recycle of hydrogen sulfide, which is similar in some respects to the Groenendaal process, is disclosed in U.S. Pat. No. 4,153,674 to Verloop et al. In this procedure, the feedstream is applied through a nonselective absorption unit and to the effluent side of a Claus sulfur recovery unit, completely bypassing the Claus unit. The effluent from the Claus unit is applied through a heater and reduction reactor to convert the sulfur values to hydrogen sulfide. The output from the reduction reactor is passed to a selective absorption unit. A recycle gas which is relatively rich in hydrogen sulfide is recovered from the absorption unit and applied to the front of the Claus sulfur recovery unit. This gas, either with or without supplementation with a hydrogen sulfide rich gas of low carbon dioxide content, provides the sole input to the Claus unit.
Another method for processing low hydrogen sulfide gas streams is disclosed in U.S. Pat. No. 4,113,849 to Atwood. In this process, a Claus unit reactor is operated at a relatively low pass yield e.g. about 65% based upon the hydrogen sulfide content, and the reactor tail gas is applied to an incinerator to oxidize the unreacted hydrogen sulfide to sulfur dioxide. The SO.sub.2 is ultimately extracted from the tail gas stream by a solvent absorption and regeneration procedure to arrive at a sulfur dioxide containing recycle gas which is mixed with the hydrogen sulfide feedstream at the input to the Claus unit. The Atwood process is said to be self balancing in the sense that, if the yield in the Claus reactor exceeds 67%, a less than stoichiometrically equivalent amount of sulfur dioxide will be contained in the recycle gas, thus retarding the Claus reaction to a maximum of 67%. In addition, the Atwood reference discloses splitting of the feedstream to divert a portion thereof directly to the incinerator for direct combustion to sulfur dioxide, thus increasing the sulfur dioxide concentration in the recycle gas.